Fuel Cells were invented about 170 years ago by Sir William Grove. Nonetheless, the promise of generating electricity at high efficiency from the electrochemical reaction of a fuel and oxygen has remained elusive. Widely used technologies, such as combustion of fossil fuels or heat from nuclear reactors, rely on thermal cycles to generate stream to power turbines and connected generators. Because the efficiency of these systems is limited by the second law of thermodynamics to the Carnot limit (max efficiency=(Thot−Tcold)/Thot), current power plants are about 35% efficient, rejecting about 65% of the heat to the environment. This efficiency is limited by the materials available that can reliably withstand Thot and last for many years of operation. Technologists believe that even the most advanced materials possible can only extend the efficiency of thermal power plants to 50% at best. However, while not a common practice, the overall efficiency can be increased by using the waste heat in co-generation systems.
In contrast, fuel cells are the only known method of potentially converting fuels to electricity at nearly 100% efficiency. The free energy of any redox reaction could theoretically be completely converted to electrical energy. In thermal systems, efficiency is determined from the fraction of the enthalpy of the reaction that is converted, as limited by Carnot considerations and inefficiencies in the process. A direct comparison of the efficiencies of the two different systems (thermal and fuel cell) must include the difference between the free energy and the enthalpy: namely TΔS (T is the absolute temperature and ΔS the change in entropy).
Because of the promise of high efficiency, many potential fuel cell technologies have been explored and are under development. These technologies are characterized by the temperature of operation and also by the fuels that can be used in that technology. One such technology is Polymer Electrolyte Membrane Fuel Cells (PEMFC) that operates at or near room temperature. These fuel cells currently are being developed for possible use in automobiles and for a variety of portable applications. There are still many challenges to be overcome if any of the fuel cell technologies are to be widely deployed, including PEMFCs.
Current PEMFC technology uses carbon black as a catalyst support. The particular blacks used have been optimized to bind 3 to 5 nm platinum catalyst nanoparticles and are conducting enough to transport charge to and from the catalyst. The carbon morphology is also important in supporting open porosity in the electrodes, so that fuel or oxidant can enter and product gasses can escape. Typically, the currently utilized carbon black (e.g., Vulcan XC-72) forms an open interconnected network of 50-100 nm particles.
However, the carbon black catalyst support corrodes too rapidly, especially under transient load and on/off operation conditions. Carbon, in any form, is thermodynamically stable below about 0.2 V, but only kinetically stable above that potential (C+2H2O→CO2+4H++4e−, E°=0.207 V). However, under fuel starvation in a fuel cell stack, even the anode of a single cell can be forced to a high potential (up to +1.5 V) as oxidation of the carbon is the only process that can support the imposed stack current. In fact, no single metal is thermodynamically stable at such potentials in acidic aqueous media. Gold has the highest reduction potential at 1.5 V vs SHE and will dissolve at lower potentials if the concentration of gold cations in solution is low—as it would be in a PEMFC. Of course, the expense of gold precludes it being used as a catalyst support if fuel cells are to be widely used. Rather, metals either dissolve as cations or less frequently form chemically passivating and electrically insulating oxide coatings. Under alkaline conditions, soluble metal-containing oxo-anions or passivating oxide/hydroxide layers usually result. Since the free energy of formation of intermetallic compounds is usually small compared to that of the respective oxides of the same metals, the result is not much different when intermetallic compounds are exposed to such highly oxidizing conditions. The mechanisms of oxidation will likely be more complex for intermetallics (e.g., leaching and oxidation of the more electropositive element is likely to occur first) but the end result is expected corrosion.
Based on the foregoing there exists and ongoing and unmet need for a conductive catalyst support materials which can be used in fuel cell applications.